Organic light emitting devices (“OLEDs”) have shown great promise as thinner, lighter-weight displays for the displacement of current liquid crystal displays (“LCDs”). This is due at least to the lower power consumption, wider view angle, better brightness, video-speed capability and simpler and lower cost manufacturing processes of OLEDs relative to LCDs. An OLED is a device that utilizes an organic species (either a small molecule or a polymer) to emit light under an applied electric field. Many different OLED architectures are known, but all typically share some common features. For example, OLEDs typically include an anode formed from a transparent electrically conductive material (for example, indium tin oxide (“ITO”)), a cathode formed from a low work function metal (for example, lithium, magnesium, indium, calcium, or barium), and two or more organic layers disposed between the cathode and the anode. Application of an electric field across the cathode and anode causes electrons and holes respectively to be injected into the organic layers and move through the device. The holes and electrons may combine in the organic layers to form excited molecular species (“exitons”), which may then emit light via decay to the ground state. Emitted light can exit the OLED through the transparent anode.
In so-called frontside-emitting OLEDs, the ITO layer is deposited on a transparent substrate, such as glass, and the organic layers are deposited onto the ITO layer. Emitted light exits the device through the substrate. In so-called backside-emitting OLEDs, the ITO layer is deposited after the organic light emitting layer. Emitted light exits the device through the face opposite the substrate.
As mentioned above, OLEDs potentially offer many advantages over other display technologies. For example, OLEDS are inexpensive and relatively simple to manufacture, do not have a brightness that is dependent upon viewing angle, and can potentially be formed on flexible substrates. However, the low work function metals typically used as cathodes in OLEDs are environmentally sensitive, and may be easily oxidized by moisture or oxygen from air. Therefore, it is important to protect these materials from air both during processing and during use.
Commercially-produced OLEDs typically utilize a glass or metal canister that is bonded over the device layers of the OLED with an adhesive to protect the device from atmospheric water vapor and oxygen. A desiccant may be included within the canister interior for additional protection. Such structures are quite bulky, and therefore are suitable only for smaller applications, such as for personal digital assistants (“PDAs”) and cellular phones. Furthermore, the rigidity glass or metal canister also may make these encapsulation methods unsuitable for flexible OLEDs.
Another approach for encapsulating OLEDs has been to deposit alternating films of polymer and inorganic materials over the OLED device layers. For example, U.S. Pat. No. 6,570,325 to Graff et al. discloses depositing alternating layers of a polymer, such as polyacrylate or parylene, and an inorganic material, such as aluminum oxide, silicon dioxide, silicon nitride, etc. over the device layers. In this structure, the inorganic layer acts as a moisture/oxygen barrier, while the organic material serves to “decouple” adjacent inorganic layers, thereby preventing defects in the inorganic layers (which are the primary route of oxygen/water vapor transport through the inorganic layers) from propagating between layers. Such a barrier stack utilizing polyacrylate as a polymer layer has been found to have a water vapor transport rate (“WVTR”) of approximately 10−5-10−6 g/m2/day/atm. This is adequate for devices having expected lifetimes of two years or less, but not suitable for devices with expected lifetimes of more than two years, such as computer monitors and televisions, which require a WVTR on the order of 10−10-10−12 g/m2/day/atm.
Polyacrylate has become the most common commercially used polymer for such barrier stacks. This is because the polyacrylate can be formed by thermally evaporating polyacrylate monomer onto a substrate under vacuum, followed by irradiating the acrylate monomer with UV radiation to form the polymer. However, several critical issues have prohibited this process from becoming a production-worthy process for protection of OLED devices. For instance, in the barrier stack of Graff, the polymer layers have relatively high WVTR and oxygen transport rate (“OTR”) values, and therefore offer little assistance to the inorganic layers in blocking oxygen and water vapor. The films deposited as described in Graff are amorphous, and tend to have disordered, porous structures, leading to the high WVTR and OTR values. Furthermore, the monomer used to form polyacrylate tends to deposit on all surfaces of a deposition chamber, rather than just the substrate. Therefore, the deposition chamber used to deposit the polyacrylate may require frequent cleaning. Also, the acrylate monomer is a liquid, and can penetrate the organic light emitting molecules of the OLED during encapsulation via pinholes in the cathode. This may render the organic light emitter less effective. Additionally, the polyacrylate layer has limited thermal stability. Therefore, deposition of an inorganic barrier layer over a polyacrylate layer requires the use of lower temperature processes such as sputtering, which typically have lower throughputs than the deposition of such films by chemical vapor deposition. Furthermore, polyacrylate has a much higher coefficient of thermal expansion (>30-50 ppm/° C.) than commonly used inorganic films (2-6 ppm/° C.). This large difference in the CTE of the inorganic and polyacrylate layers may result in high thermal stress at the interfaces between layers. Since multiple alternating polyacrylate and inorganic layers are needed for achieving a suitably low WVTR, the resulting high thermal stresses at the inorganic-polyacrylate interfaces may cause delamination of the layers, making the reliability of such OLED displays an issue for long-term applications.